University of Colorado study finds that bumps travel

When bumps are soft and troughs are clean, they turn mere mortals into undulating gods (thus the term “hero bumps”). But when the light is flat and the mounds are diamondhard lumps of ice, moguls become the harbingers of faceplants and the first step in funding your reconstructive knee surgeon’s vacation to Acapulco. It turns out they have another trick up their snowy sleeves: they like to move uphill.

A trio of Colorado scientists recently put their hypothesis to the test, namely that moguls tend to migrate uphill. Before you think this was a drunken bet dreamed up in the dizzy altitude of a midmountain bar, keep in mind the apocryphal drunkards have some serious kinetic authority. Ted Pfeffer is a professor of civil, environmental and architectural engineering at CU; David Bahr is a Regis University professor and studied geological sciences at CU; Ray Browning is a professor at Colorado State University.

Beyond merely speculating that moguls are on the move, the three recently published an article in the November issue of Physics Today, titled “The Surprising Motion of Ski Moguls.” You can check it out after perusing the article on theoretical femtoseconds.

Common sense and physics dictate that skiers push into their turns as they hit the low point in the trough, stripping snow from the top bump and building up the one below it. With each incremental transfer, the entire mogul field enters a process known as backward propagation. Pfeffer uses even fancier language: “Moguls are a type of kinematic wave, an entity rather different from the more commonly studied dynamic wave. The classic example of kinematic waves, and the setting in which kinematic wave theory was formulated, is the flow of cars on a highway.”

In plain language, when that idiot in the rental Tahoe cuts you off while merging onto I-70, it causes a backward propagating wave that causes every car behind it to bunch up, severely impeding your chance of getting home in time to watch the Danish team compete in the Olympic semifinals of women’s curling. So it goes with moguls.

As it turns out, the top base of a mogul can move as much as 8 centimeters a day and 10 meters uphill a season, which means the mogul that rag-dolled you into the air in December can do the same job 30 feet higher up the hill in April. While mogul mountain minutia may seem like a flakey thing to study — pun intended — there is a bigger picture for this type of research.

Pfeffer used his study to test a type of time-lapse photography known as photogrammetry, a scientifically visual way of measuring objects in motion. So besides settling some sort of nerdy bet, these three are also helping save the world. Pfeffer is now using time-lapse photogrammetry with the Extreme Ice Survey to study and document the retreat of glaciers and snowfields (visit some of the amazing results at www. extremeicesurvey.com).